This invention relates generally to sonar systems, and more particularly to synthetic aperture sonar particularly useful for bottom mapping, and the like.
Two requirements in a practical bottom mapping sonar are high resolution and a large area coverage rate. A variety of towed vehicle carried, side-looking sonar systems have been implemented or proposed, including, synthetic aperture sonars which have been shown capable of increasing resolution. The synthetic aperture sonar differs from conventional fixed aperture sonars in that the horizontal aperture of the synthetic aperture sonar depends upon horizontal motion of the vehicle carrying the receiving element. It has further been shown that by using multiple receiver elements in a vernier array, as described in U.S. patent application Ser. No. 776,800, filed Mar. 4, 1977, by Chester C. Loggins, Jr. and assigned to the assignee hereof, the effective along-track sampling rate can be increased, resulting in an increase in area coverage rate by a factor substantially equal to the number of receiving elements.
There are, of course, practical limits to the number of elements of a vernier synthetic aperture array. These include, but are not necessarily limited to, vehicle size, signal processing requirements, and cost. Accordingly, other approaches to increasing still further the resolution and area coverage rates of synthetic aperture sonars have been sought.
Area coverage may be considered to be dependent at least in part upon the speed of travel of the vehicle and on the range capability of the sonar, while resolution is dependent in part on sonar signal frequency and pulse repetition rate, all of which must take into consideration the limiting factor of the low velocity of sound in water.